# Connect a antenna with 75Ω line impedance to VNA

1. Jan 12, 2009

### N468989

Connect an antenna with 75Ω line impedance to VNA

Hello, how can i connect an antenna with 75Ω line impedance if vna has 50Ω input impedance.

thanks

Last edited: Jan 13, 2009
2. Jan 12, 2009

### Staff: Mentor

The traditional way is with a balun or other matching scheme. What frequency range? What type of antenna? Pointer to the VNA spec?

3. Jan 12, 2009

### N468989

It's a half-wave dipole antenna, the frequency range is aprox 2.4Ghz to 2.5 Ghz. The VNA don't know which one yet.

For the balun can make one with normal components?

What other matching schemes could i do?

4. Jan 12, 2009

### Staff: Mentor

Since you need to also do the Balanced-Unbalanced translation (dipole-to-coax), the balun would be the way you'd normally do that. There are plenty of commercially available parts for that ISM band. I did a google search for ism band balun, and got lots of good hits. Here's a nice paper:

http://www.rfdesignline.com/howto/197006085

You can also make baluns with lengths of coax, as shown for a specialized ISM antanna in this article:

http://pe2er.nl/wifisector/

The ARRL Antenna Books also show different ways to make baluns for dipole antennas.

5. Jan 12, 2009

### Pumblechook

That is not a bad match actually. SWR is 1.5. Many systems work on a SWR up to 2 so I wouldn't worry bother matching any closer.

Baluns are for converting balanced to unbalanced and not really matching devices.

6. Jan 12, 2009

### f95toli

What are you trying to measure? For most measurements the mismatch shouldn't be too much of a problem; most of the the reflected power will be absorbed inside the VNA to standing waves etc shouldn't be too much of a problem.
The "proper" way of doing this is to set the characteristic impedance to 75 Ohm in the calibration menus of the VNA and then use a 75 Ohm calibration kit. However, these are rare and quite expensive.

You could try de-embedding the DUT using a pad, but that is quite tricky; at least if you need an accurate measurement.

Also, be careful when you connect your device to the VNA; 75 Ohm connectors DO NOT FIT on 50 ohm systems; you MUST use an adapter (or a minimum loss pad or something similar) or you WILL destroy the connectors on the VNA.

7. Jan 12, 2009

### yungman

Buy one if you can. Nothing is simple dealing with RF!!!

You can always make a quarter wave impedance transformer. Fab it out of a 2 sided FR4 ground plane boards. Edge out a microstrip of quater wave length at 2.45Ghz. The impedance should be $$\sqrt{75X50}$$ which come out to be 61.2ohm. Look up the wave length on FR4. find the dielectric thickness and look up the width of the trace. The length should be about .6 inches or so?? small enough to do it in a small board. You can then use the VNA to de embard and match. That is if you want to do it as a charllenge!!! Buy one if you can!!!

8. Jan 12, 2009

### N468989

I will measure the resturn loss (S11) for example. How can i adapt the line impedance for the VNA (i.e build a circuit)?

If i fit a 50Ω conector on the 75Ω cable will it be ok or are there any problems?

so if i buy a balun for 75Ω->50Ω i'm ok?

I understand what you mean but that may be a bit to difficult to make that. i will search that also!

thankyou

9. Jan 12, 2009

### yungman

If you really want something NOW. You can always use a two components matching. Use Smith chart to design. You have to use surface mount cap and inductor. This kind is the fastest and easiest to build but is very fragile, you bend the coax and you break the component easily. You need help using smith chart? Also you need to deal with double end to single end. But the circuit board is so small, I think you just do it and it will be fine!!!

Yes buying one is the way to go in the long run.

10. Jan 12, 2009

### Corneo

I second f95toli's suggestion.

11. Jan 13, 2009

### N468989

this is good advice thanks! i will check this.

Yes of course i will use the right BNC connectors. But is it ok to connect the 75 Ohm cable to the VNA? Pumblechook stated that SWR is 1.5 so it's ok.

12. Jan 13, 2009

### f95toli

Well, the VNA won't care as long as you use 50 Ohm connectors. All VNAs can measure loads with whatever impedance you like (almost) and you will get accurate results as long as you calibrate them using a calibration kit of the right impedance; this is why you can set the characteristic impedance of your kit in the calibration menu. The fact that that VNAs have 50 Ohm connectors is mainly due to the fact that this is the most common impedance in high-frequency applications.

Hence, your problem is simply that you (presumably) only have access to a 50 ohm calibration kit; this is why you need e.g. a balun or some other matching network.

Note that if you are only interested "rough" results (e.g. seeing where the antenna radiates the most) you might even be able to get away with a DIY calibration kit; i.e. make a 75 ohm load using a SMD resistor; the open and short are more problematic but you could always try just using a piece of open 75 ohm cable and a shorted one:tongue: , the reference planes won't be in the right place but I suspect this might still be more accurate than trying to build a DIY matching network; especially since you have no accurate way of de-embedding the latter.
But buying a balun or (better) borrowing a 75 ohm calibration kit from someone would be the best solution.

13. Jan 13, 2009

### N468989

14. Jan 13, 2009